1. Field of the Invention
The present invention relates to reservoir technology, and more particularly to a method of acquiring information of hydraulic fracture geometry for evaluating and optimizing well spacing for a multi-well pad.
2. Description of Background Art
Over the years, the research on reservoir technology focuses on maximizing the value of ultra-tight resources, sometimes referred to as shales or unconventionals resources. Ultra-tight resources, such as the Bakken, have very low permeability compared to conventional resources. They are often stimulated using hydraulic fracturing techniques to enhance production and often employ ultra-long horizontal wells to commercialize the resource. However, even with these technological enhancements, these resources can be economically marginal and often only recover 5-15% of the original oil in place under primary depletion. Therefore, optimizing the development of these ultra-tight resources by optimizing the well spacing and completions is critical.
In conventional oil fields, there are many methods used for attempting to optimize well spacing. One of the most common methods is downspacing tests, where varying well spacings are chosen for different pads and production is compared at different spacings to assess which spacing is optimal. This technique is expensive and time consuming and often gives a highly uncertain answer, requiring this procedure to be repeated many times to increase accuracy in the result. This procedure, which often ends up with under drilling and over drilling numerous pads, can significantly reduce the value of the resource due to inefficient development. Another technique which has been widely adopted is to use subsurface or surface micro-seismic arrays to monitor seismic events during the hydraulic fracturing process. Ideally, this would provide insight into the dimensions of hydraulic fractures, helping to determine the optimal well spacing. However, this technology is often questionable for a number of reasons. First, and foremost, it is often accepted that microseismic predominantly identifies shear events, which may or may not be associated with the growth of hydraulic fractures. A second challenge with microseismic is that it requires knowledge of the subsurface, particularly wave velocities in the media, which are often unknown and have high uncertainty. Finally, the processing methods themselves are often brought into question, as many service companies who provide this technique use veiled algorithms and openly admit the uncertainty in these processing methods. Despite all these uncertainties and the significant cost of running microseismic, the value of understanding well spacing is so great that this technique has been widely applied in industry. Further, there are newer approaches under development which utilize advanced proppants or advanced imaging and data acquisition techniques. However, these approaches are still in the research stage and will likely be quite costly and potentially complex even if they are commercialized.
Another technology which has been used to evaluate well spacing is pressure measurements. This technology has been done downhole and at the surface. Tests have been performed during production, during shut-ins, and during hydraulic fracturing. For ultra-tight systems, tests during production are rarely done, even though that is the most commonly employed method for conventional systems to evaluate reservoir performance or fracture geometry. The shut-in times and data acquisition times for unconventional resources are often too long to justify these tests. Downhole gauges can be extremely expensive, particularly when placed anywhere along the lateral of a horizontal, costing sometimes in excess of 1 million dollars per gauge, particularly in unconventional resources, which are often deep formations, sometimes greater than 10,000 ft in depth. In addition, retrievable downhole gauges have been used, but again these gauges only measure pressure at one location in the well and can be quite costly to install and retrieve. Moreover, they cannot be used during the hydraulic fracturing process very easily, although some newer technologies are coming out to solve this problem. Because of the cost limitations of any method of measuring downhole pressures, the industry is slowly recognizing that surface gauges can be useful during the hydraulic fracturing process since there is a single, known, stable phase in the wellbore, allowing for surface gauges to act as surrogates for downhole gauges during the hydraulic fracturing process. Several tests have been done where surface gauges have been used during hydraulic fracturing. However, these tests do not involving isolating portions of wells off and thus the surface gauges are only measuring the response in the entire well of hydraulic fracturing operations in adjacent wells.
To date, no methods for evaluating hydraulic fracture geometry and optimizing the well spacing with less cost, more accurate results, and much fewer wells and inefficiently developed pads compared with the above mentioned conventional methods, have been successfully deployed in ultra-tight oil resources. Therefore, there is an industry-wide need for a method for evaluating hydraulic fracture geometry and optimizing well spacing for a multi-well pad in order to better understand optimal well spacing, so as to maximize the value of ultra-tight resources with less cost and higher certainty.